Head to Arkadia. Brasidas and Myrrine will be at the Statue of Artemis in The Cedar. It's on the border of Smuggler's Forest and the Golden Fields.

Upon arriving at the statue, Myrrine and Brasidas will not be there. Instead, a mercenary will be. No matter how the conversation goes, he will attack. Dodge when he turns red, and use abilities to take him down. Because of bounties, several other mercenaries may come along.

At the end of the battle, he will beg for mercy. Kill him or let him live--this choice will not have long-term effects. If he lives, he gives information about Lagos.

Run southwest of the statue to the Tegea. Myrrine and Brasidas will be standing at the top of a cliff near some tents. Speak to them. Brasidas will have one plan, and Myrrine will have another. Brasidas wants to talk in a farmhouse nearby, and Myrrine suggests taking him down by burning silos and killing guards to loosen his hold on the region.

After these conversations, Lagos will be revealed as a cultist and the mission will come to an end.